Make the shortcrust pastry: sift the flour and salt into a bowl. Rub in the butter cut into small pieces until the mixture resembles fine breadcrumbs. Add the cold water and mix to a dough. Chill for 15 minutes before using to line a 24cm flan tin.
Preheat your oven to 200°C.
Mix the filling ingredients together.
Place the plums cut side up onto the flan case. Pour the filling on top.
Put into the oven for 20 minutes.
Mix the demerara sugar and the mixed spice and sprinkle onto the flan. Put the flan back in the oven at 230°C for another 20 minutes until golden on top.
Leave to cool for a few minutes before serving or serve cold with vanilla ice-cream.